When boreholes are drilled so that explosive charges may be delivered to an underground geological structure, it is important that the charges be placed at appropriate levels in the borehole. Where more than one explosive deposit is to be placed at spaced intervals in a borehole, it has been conventional to apply a column of concrete or the like to a portion of the borehole so that a subsequent explosive charge may be spaced at a distance from, for example, the bottom of the bore hole. The application of concrete or the like to this region of the bore hole is both time consuming and expensive. Further problems arise if the borehole is partially or completely full of water. Some of these problems can be overcome by placing what is known as decking at selected depths in the bore hole.
Different types of decking are known in the art. They may comprise a wooden or concrete disk having a cross-section substantially corresponding with the bore cross-section and lowered by rope to a water surface. It has also been proposed to use as a decking a polyester resin which floats on the water and solidifies after a period of time. More recently, it has also been suggested to replace the decking with a gel-explosive slurry which floats on the water layer. Each of these methods suffer disadvantages. More recent methods are described in Australian Patents Nos. 579 395 and 595 887, the contents of which are incorporated herein by cross-reference. Australian Patent 579 395 describes a borehole plug composed of two or more co-reagents which expand when mixed, separately contained so that they can be mixed when so required and further contained in an outer container adapted to retain the co-reagents during mixing. The container is dimensioned for dropping or lowering down a bore hole. Typically co-reagents react to form a polyurethane foam.
Australian Patent No. 595 887 describes an improvement on that system involving reagents which when mixed create a gas which expands in the container to position the plug at the level in the borehole to which the device has been lowered.
In expansion-type borehole plugs of the type described in Australian Patent Nos. 579 395 and 595 887, the time required for the plug to become fully expanded or inflated is dependent on the rate of reaction of the components of the reactive system employed. During the time before expansion has proceeded far enough to cause the borehole plug to be fixed in position it must be lowered or otherwise located at the desired position. However when the temperature of the reactants varies, the rate of their reaction also varies, often to an unpredictable extent. Borehole temperatures vary depending on their depth, location and the climatic conditions. Thus, it has been found that the expansion-type borehole plugs known to the art are difficult to use in practice because the time required to inflate or expand the plug is difficult to control, making for difficulty in locating the plug at the desired depth in the borehole. In addition, in cold environments reactions causing the release of inflating gases may be very slow, leading to operating delays or even failure of plugs to be effectively fixed in position.
Furthermore, the shelf-life of expansion-type borehole plugs known to the art may be limited owing to the aggressive nature of the chemical reagents involved and the likelihood of physical damage to the containers of the reactive components during transport and storage.
It is an object of the present invention to ameliorate or substantially overcome these difficulties inherent in known borehole plugs.